Method for fabricating electrical circuitry on ultra-thin plastic films

ABSTRACT

In accordance with the teachings of one embodiment of the present disclosure, a method of forming high-density metal interconnects on flexible, thin-film plastic includes laminating a dry photoresist layer to a substrate. The photoresist-laminated substrate is baked. An assembly is formed by laminating a plastic film to the baked, photoresist-laminated substrate. One or more electrically conductive interconnect layers are processed on a first surface of the laminated plastic film. The processing of the one or more electrically conductive interconnects includes photolithography. The assembly is baked and soaked in a liquid. The processed plastic film is then separated from the substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119(e)of U.S. Provisional Patent Application Ser. No. 60/892,678 entitled“Method for Fabricating Interconnect on Ultra Thin Plastic,” which wasfiled on Mar. 2, 2007.

GOVERNMENT FUNDING

This invention was made with Government support under Contract No.F33615-99-C-1513 awarded by the United States Army. The Government mayhave certain rights in the invention.

TECHNICAL FIELD

This disclosure relates in general to electronics, and more particularlyto forming electrical circuitry on ultra-thin plastic films.

BACKGROUND

Photolithography (also optical lithography) is a process used insemiconductor microfabrication to selectively remove parts of a thinfilm or the bulk of a substrate. Photolithography typically uses lightto transfer a geometric pattern from a photomask to a light-sensitivephotoresist on substrate. A series of chemical treatments then engravesthe exposure pattern into the material underneath the photoresist. Insome complex integrated circuits, such as modern Complementarymetal-oxides semiconductor (CMOS), a semiconductor wafer will go throughthe photolithographic cycle as many as fifty times. Usingphotolithography to form high-density patterns often requiresparticularized surfaces, such as extremely flat surfaces that areinsensitive to elevated temperatures, due to process requirements andtolerance constraints.

SUMMARY

In accordance with the teachings of one embodiment of the presentdisclosure, a method of forming high-density metal interconnects onflexible, thin-film plastic includes laminating a dry photoresist layerto a substrate. The photoresist-laminated substrate is baked. Anassembly is formed by laminating a plastic film to the baked,photoresist-laminated substrate. One or more electrically conductiveinterconnect layers are processed on a first surface of the laminatedplastic film. The processing of the one or more electrically conductiveinterconnects includes photolithography. The assembly is baked andsoaked in a liquid. The processed plastic film is then separated fromthe substrate.

Some embodiments of the present disclosure may enable the fabrication ofsophisticated electronic circuitry on transparent, plastic films havinga thickness within the ranges of 10 to 500 microns; however, the plasticfilms may have any suitable thickness. In addition, the circuitry formedon the plastic film of some embodiments may have widths within the 5 to50 micron range, though any suitable width may be used. Some embodimentsmay provide multi-layer interconnects, including, for example,interconnected circuitry on both sides of a thin plastic film. In someembodiments, the circuitry may be able to perform particular functions,such as, for example, transmitting signals. In addition, in someembodiments the electronic circuitry may be invisible to the naked eyeand thus a fully processed plastic film may appear substantiallytransparent.

Other technical advantages of the present disclosure will be readilyapparent to one skilled in the art from the following figures,descriptions, and claims. Moreover, while specific advantages have beenenumerated above, various embodiments may include all, some, or none ofthe enumerated advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and itsadvantages, reference is now made to the following description, taken inconjunction with the accompanying drawings, in which:

FIG. 1A shows a cross-sectional view of a portion of a substrate afterthe application of an adhesive to an outer surface of the substrateaccording to one embodiment;

FIG. 1B shows a perspective view of a portion of the substrate of FIG.1A after the adhesive has bonded a plastic film to the substrate andafter the formation of electronic circuitry on a surface of the plasticfilm;

FIG. 1C shows a perspective view of a portion of the substrate of FIG.1B during a process that removes plastic film from the substrate afterthe electronic circuitry has been processed on the surface of theplastic film; and

FIG. 2 is a flow chart illustrating a method of forming interconnects onthin film according to the teachings of one embodiment of the presentdisclosure.

DETAILED DESCRIPTION

The present disclosure provides example methods for fabricatinghigh-density electronic circuitry on ultra-thin plastic films. Ingeneral, certain methods include the following acts: (i) an ultra-thinplastic film is laminated to a substrate; (ii) high-density circuitry ismicro-fabricated on a surface of the plastic film usingphotolithography, metal depositions, and etching; and (iii) the plasticfilm is separated from the substrate. The example embodiments of thepresent disclosure are best understood by referring to FIGS. 1A through2 of the drawings, like numerals being used for like and correspondingparts of the various drawings.

FIG. 1A shows a cross-sectional view of a portion of a substrate 100after the application of an adhesive 102 to an outer surface of thesubstrate 100 according to one embodiment. Substrate 100 generallyrefers to any surface that enables the fabrication of electroniccircuitry on ultra-thin plastic films coupled to the substrate 100surface. Substrate 100 may be formed from any suitable material and mayhave any suitable dimensions. For example, substrate 100 may be anoxidized silicon wafer with a 150 millimeter diameter or a square 12×12inch glass substrate. In various embodiments, substrate 100 may belarger or smaller depending on the particular application and/or thecapabilities of subsequent processing.

Adhesive 102 generally refers to any material capable of bonding, atleast temporarily, a plastic film to substrate 100. In this example,adhesive 102 is a dry photoresist film that may be laminated tosubstrate 100. One example of such a film is the Riston® FX900 Seriesphotoresist film made by Dupont. Dry photoresist films often include aprotective cover 104, such as a Mylar film, designed to shield theunderlying photoresist film from exposure to light or the elements. Asexplained further below, the protective cover 104 may itself provide asuitable plastic surface for subsequent circuitry processing, therebysimplifying the process flow of certain embodiments. The limitedtransparency level of some protective covers 104, however, is notsuitable for some applications. In this example, protective cover 104 isremoved in preparation for further processing.

Although this example uses a laminated photoresist film 102 to bondplastic film to substrate 100, any suitable adhesive 102 or othersuitable bonding force may be used. For example, in some alternativeembodiments static electricity may sufficiently bond a thin plastic filmto substrate 100. In some other alternative embodiments, a spray-on orspin-on photoresist or a spin-on epoxy may be used in place ofphotoresist film 102. Some types of spray-on photoresist may assure thatplastic films, such as polyethylene, can be easily removed withoutadditional stretching of the plastic film.

In this example, the photoresist-laminated substrate 100 is heated in anoven after protective cover 104 is removed. The oven bake generallyremoves some of the solvents in the exposed photoresist film 102 thatmight otherwise outgas and warp the photoresist surface. Suitable bakeparameters for this example include exposure to a temperature range of90 C to 150 C for approximately ten minutes; however, any suitable bakeparameters may be used. An ultra-thin plastic film is then applied tothe substrate, as described further with reference to FIG. 1B.

FIG. 1B shows a perspective view of a portion of the substrate 100 ofFIG. 1A after adhesive 102 has bonded a plastic film 106 to thesubstrate 100 and after the formation of electronic circuitry 108 on asurface of plastic film 106. Plastic film 106 generally refers to anyplastic material that may be bonded to substrate 100 and that provides asuitable surface for the fabrication of electronic circuitry 108. Anysuitable material or application process may be used, depending on theparticular application.

In various embodiments, plastic film 106 may have a thickness within therange of 10 to 50 microns, though plastic film 106 may have any suitablethickness that enables the fabrication of circuitry 108 on a surface ofplastic film 106 and the subsequent removal of plastic film 106 fromsubstrate 100 (e.g., some embodiments may include plastic films 106 thatare less than 10 microns thick, other embodiments may include plasticfilms 106 having a thickness confined to a narrower range of 18 to 50microns, still other embodiments may include plastic films 106 having athickness within the range of 50 to 500 microns, and particularembodiments, depending on the application, may include plastic filmshaving a thickness of 1 mil or less). In this example, a transparent, 18micron thick plastic film 106 is applied to the photoresist-laminatedsubstrate 100 by running the substrate 100 through a plastic-filmlaminator. During this lamination act, plastic film 106 is held taut,the rollers of the laminator are heated to approximately 30 C-100 C, andadhesive layer 102 bonds plastic film 106 to the substrate 100,resulting in a smooth, plastic-laminated coating of thephotoresist-laminated substrate 100.

In this example, the plastic-laminated substrate 100 is then placed inan oven and heated to a temperature that is greater than the highesttemperature exposure of plastic film 106 during subsequent electricalcircuitry processing. This oven process may pre-stretch and/orpre-shrink plastic film 106, thereby preconditioning plastic film 106before subsequently forming circuitry 108. After laminating plastic film106 to substrate 100, it may be possible to heat plastic film 106slightly above its melting temperature without destroying the plasticfilm 106. The plastic-laminated substrate 100 then undergoes electricalcircuitry 108 processing.

Electrical circuitry 108 generally refers to any conductive or resistiveinterconnects, interconnect layers, electrical components, or otherstructures that may be formed on a surface of plastic film 106. Invarious embodiments, the processing of electrical circuitry 108 may besubstantially similar to standard semiconductor processing. Suchelectrical circuitry 108 processing may include, for example, depositionof conductive or resistive films, photolithographic patterning, etching,or any other suitable processing that may be used to form circuitry 108on a surface of the plastic-laminated substrate 100. Although someembodiments may be scalable with current and future processingtechniques, some embodiments using conventional processing techniquesmay include electrical circuitry 108 that is approximately 6 micronswide with approximately 8 micron spacing between elements of theelectrical circuitry 108; however, any suitable dimensions and spacingmay be used, including, for example, electrical circuitry 108 havingwidths of 10 microns or less, or electrical circuitry 108 having widthswithin the range of approximately 2 microns to 50 microns or greater(e.g., 100 microns, 500 microns etc.) and a spacing between electricalcircuitry 108 elements of approximately 2 microns or greater (e.g., 10microns 50 microns, 100 microns, etc.). At the completion of theelectrical circuitry 108 processing, plastic film 106 is separated fromsubstrate 100, as illustrated in FIG. 1C.

FIG. 1C shows a perspective view of a portion of the substrate 100 ofFIG. 1B during a process that removes plastic film 106 from substrate100 after electronic circuitry 108 has been processed on a surface ofplastic film 106. Plastic film 106 may be removed from substrate 100using any of a variety of processes. For example, one method includessoaking the plastic-laminated substrate 100 in water for approximatelyan hour and then gently peeling plastic film 106 from substrate 100;however, any suitable soak or other removal technique may be used,including either mechanical or manual removal techniques. Forparticularly well-attached plastic films 106, the soak may alternativelyor additionally include a solvent, such as, for example, acetone, tohelp separate plastic film 106 from substrate 100. After separation ofplastic film 106 from substrate 100, a solvent clean may remove anyresidual dry photoresist film 102 from plastic film 106. Air dryingsubsequently dissipates the solvent clean. This completes the processingof the example embodiment.

Thus, according to the teachings of some embodiments of the presentdisclosure, complex circuits may be formed on the surface(s) ofultra-thin, flexible plastic films that have a thickness of 50 micronsor less; however, as disclosed previously, plastic film may have anysuitable thickness, including, for example, a thickness of approximately1 mil. The processes of various embodiments are generally applicable tostandard semiconductor fab environments. In addition, the processesdescribed herein may still apply to future scaling advances insemiconductor interconnect processing. Some embodiments may providemulti-layer interconnects, including, for example, interconnectedcircuitry on both sides of a thin plastic film. In some embodiments, thecircuitry formed on thin plastic films may be able to perform particularfunctions, such as, for example, transmitting signals. In addition, insome embodiments the electronic circuitry may be invisible to the nakedeye and thus a fully processed plastic film may appear substantiallytransparent. Additional details regarding the processing of electroniccircuitry 108 on thin-film plastics 104 or 106 are described below withreference to FIG. 2.

FIG. 2 is a flow chart 200 illustrating one example method of forminghigh-density electronic circuitry on ultra-thin plastic films accordingto one embodiment. In general, flowchart 200 includes the followingacts: (i) an ultra-thin plastic film is coupled to a substrate; (ii)high-density circuitry is micro-fabricated on a surface of the plasticfilm using photolithography; and (iii) the plastic film is separatedfrom the substrate.

In act 202, an adhesive is applied to a substrate. In this example, theadhesive is a negative FX930 dry photoresist film that includes a Mylarprotective cover; however, any suitable adhesive may be used. After thephotoresist film is laminated to the substrate, the protective cover isremoved from the photoresist film.

The photoresist-laminated substrate is then baked in act 204. The bakegenerally removes some of the solvents in the exposed photoresist film102 that might otherwise outgas and warp the photoresist surface.Suitable bake parameters for this example include exposure to atemperature range of 90 C to 150 C for approximately ten minutes;however any suitable bake parameters may be used.

An ultra-thin plastic film is then applied to the substrate in act 206.In this example, a transparent and ultra-thin plastic film is applied tothe photoresist-laminated substrate by running the substrate through aplastic-film laminator. During this lamination act, the plastic film isheld taut, the rollers of the laminator are heated to approximately 30C-100 C, and the photoresist film bonds the plastic film to thesubstrate, resulting in a smooth, plastic-laminated coating.

In act 208, the plastic-laminated substrate is then placed in an ovenand heated to a temperature that is greater than the highest temperatureexposure of the plastic film during the subsequent electrical circuitryprocessing. This oven process may pre-stretch and/or pre-shrink theplastic film, thereby preconditioning the plastic film beforesubsequently forming circuitry on the plastic film. After laminating theplastic film to the substrate, it may be possible to heat the plasticfilm slightly above its melting temperature without destroying theplastic film.

The plastic-laminated substrate then undergoes electrical circuitryprocessing in act 210. In this example, the electrical circuitryprocessing uses photolithography equipment and processing techniquessubstantially similar to the equipment and processing techniques usedfor fabricating high-density metal lines and spaces on semiconductorintegrated circuits (ICs). More specifically, the electrical circuitryprocessing may include deposition of conductive or resistive films,patterning, etching, or any other suitable processing that may be usedto form circuitry on a surface of the plastic-laminated substrate. Insome embodiments, portions of the electronic circuitry, such asinterconnects, may be 10 microns wide or less (e.g., 8 microns wide)depending on tool capabilities and/or particular circuit designs. Inaddition, such interconnects may be spaced apart from each other by 8microns or less, for example, again depending on tool capabilitiesand/or particular circuit designs.

The processed plastic film is removed from the substrate in act 212using any of a variety of processes. In this example, theplastic-laminated substrate is soaked in water for approximately an hourprior to gently peeling the plastic film from the substrate; however,any suitable soak or other removal technique may be used, includingeither mechanical or manual removal techniques. For particularlywell-attached plastic films, the soak may alternatively or additionallyinclude a solvent, such as, for example, acetone, to help separate theplastic film from the substrate. After separation of the plastic filmfrom the substrate, a solvent clean may remove any residual dryphotoresist film from the plastic film. Air drying may be used tosubsequently dissipate the solvent clean. Flow chart 200 ends aftercompletion of act 212.

As explained previously, various alternative embodiments may havesimplified process flows. For example, embodiments that use theprotective, sacrificial layer 104 of a dry photoresist film may skipacts associated with applying a second plastic film 106 to the substrate100 using a laminator. Instead, the circuitry 108 previously describedmay be formed on a surface of the protective film 104 disposed outwardlyfrom the photoresist 102.

Additionally, various alternative embodiments may have more complicatedprocess flows. Some such embodiments may use multi-level circuitrydisposed on one side or both sides of the thin plastic film. Forexample, disposing multi-level circuitry on a first side of the thinplastic film may be effected by alternatively forming an interconnectlayer and an outwardly disposed dielectric layer using techniquessubstantially similar to those of standard semiconductor processes.Forming interconnects on a second side of the thin film may be effected,for example, by adhering the fully processed substrate to a secondsubstrate. The resulting assembly may generally include the firstsubstrate, the first adhesive, the thin film, the second adhesive, andthe second substrate. Separating the first substrate and first adhesivefrom the assembly thus exposes the second side of the thin film forsubsequent processing. At some point, the first and second sides of thethin film may be interconnected by forming vias through the thin filmusing any of a variety of methods. In one non-limiting example, laserablation may form vias at predetermined locations.

Although the present disclosure has been described with severalembodiments, a myriad of changes, variations, alterations,transformations, and modifications may be suggested to one skilled inthe art, and it is intended that the present disclosure encompass suchchanges, variations, alterations, transformations, and modifications asfall within the scope of the appended claims.

What is claimed is:
 1. A method of forming high-density metalinterconnects on flexible, thin-film plastic, comprising: laminating adry photoresist layer to a substrate; baking the photoresist-laminatedsubstrate; forming an assembly by laminating a plastic film to thebaked, photoresist-laminated substrate; processing one or moreelectrically conductive interconnects on a first surface of thelaminated plastic film, the processing of the one or more electricallyconductive interconnects comprising photolithography; baking theassembly; soaking the assembly in a liquid; and separating the processedplastic film from the substrate.
 2. The method of claim 1, wherein theplastic film is substantially transparent.
 3. The method of claim 1,wherein the plastic film has a positive thickness of 50 microns or less.4. The method of claim 1, wherein: the processed one or moreelectrically conductive interconnects each has a positive width of 10microns wide or less; and at least two of the processed one or moreelectrically conductive interconnects are spaced apart from each otherby a gap defined by a positive distance that is 8 microns wide or less.5. A method of forming high-density electronic circuitry on thin-filmplastic, comprising: coupling a plastic film to a photoresist layerdisposed outwardly from a first substrate; forming, usingphotolithography, one or more electrically conductive interconnects on afirst surface of the plastic film; and separating the plastic film fromthe first substrate and the photoresist layer.
 6. The method of claim 5,further comprising: laminating the photoresist layer to the firstsubstrate; baking the photoresist-laminated first substrate; and whereincoupling the plastic film to the photoresist layer disposed outwardlyfrom the first substrate further comprises using the laminatedphotoresist layer to adhere the plastic film to the first substrate. 7.The method of claim 5, wherein coupling the plastic film to thephotoresist layer disposed outwardly from the first substrate furthercomprises laminating the plastic film to the photoresist layer.
 8. Themethod of claim 5, wherein the plastic film has a positive thicknessthat is less than 1 mil.
 9. The method of claim 5, wherein the plasticfilm is substantially transparent.
 10. The method of claim 5, wherein atleast one of the plurality of electrically conductive interconnects hasa positive width that is 10 microns or less.
 11. The method of claim 5,wherein respective portions of at least two of the plurality ofelectrically conductive interconnects are spaced apart from each other apositive distance that is 8 microns or less.
 12. The method of claim 5,further comprising baking the first substrate after coupling the plasticfilm to the first substrate.
 13. The method of claim 5, whereinseparating the plastic film from the first substrate further comprisessoaking the first substrate in a liquid.
 14. The method of claim 5,further comprising forming one or more electrically conductiveinterconnects on each of a plurality of layers disposed outwardly fromthe first surface of the plastic film.
 15. The method of claim 5,further comprising: coupling a second substrate to the first substrate,the coupled first and second substrates forming an assembly; exposing asecond surface of the plastic film by separating the first substratefrom the assembly; and forming electrically conductive interconnects onthe exposed second surface of the plastic film.
 16. The method of claim15, further comprising forming conductive vias through the plastic film.17. The method of claim 16, wherein forming the conductive vias throughthe plastic film further comprises laser ablating a portion of theplastic film.